Method for producing sheet ingots by vertical casting of an aluminium alloy

ABSTRACT

A method for casting a metal alloy in an ingot mold extending along a vertical axis, the horizontal section of the ingot mold being parallelepiped in shape. During casting, a travelling alternating magnetic field is applied to a liquid phase of the alloy, the magnetic field having a maximum amplitude propagating along an axis of propagation. Under the effect of the magnetic field, a Lorentz force is applied to the liquid phase of the alloy, such that a Lorentz force of maximum intensity propagates along the axis of propagation. The method includes modulating the maximum intensity of the Lorentz force propagating along the axis of propagation. This modulation is obtained by varying, over time, one or more parameters, referred to as force parameters, governing the Lorentz force. An ingot obtained by the method is also described.

TECHNICAL FIELD

The technical field of the invention is the producing of ingotsfollowing a casting of a liquid aluminum alloy

DESCRIPTION OF THE INVENTION

During a vertical casting, aiming to form an ingot, the solidificationof a metal or of a metal alloy is affected by phenomena referred to asmacroscopic segregations. During the cooling of the metal, convectioncurrents are formed, generating recirculation vortexes, the latter beingat the origin of macroscopic segregations when their lifespan is of thesame magnitude as the characteristic durations of solidification. Thesephenomena lead, in the solidified ingot, to a local depletion or to alocal enriching with chemical species. These macroscopic segregations,or macrosegregations, are at the origin of heterogeneities in thecomposition of the ingot.

A macrosegregation well known to those skilled in the art is thenegative central macrosegregation, resulting from a depletion ineutectic alloy elements, along a vertical central axis of the ingot.These macrosegregations have been described in the work of John Wiley etal “Direct-Chill Casting of light alloys”, Wiley Publishing, September2013, pp 158-172.

The main mechanisms at the origin of the central macrosegregationdescribed in this work are

The thermosolutal convection in the sump caused by the temperature andconcentration gradients, and the penetration of these convective flowsinto the pasty zone;

The transport of grains in the supercooled zone under the effect ofgravity, the Archimedes buoyancy principle and natural or forcedconvection;

The flow in the pasty zone solicited by the volumetric shrinkage atsolidification, which can be assisted by the metallostatic pressure;

The flow of the liquid in the pasty zone caused by mechanicaldeformations;

The forced flows that can result from the pouring, injection or from arelease of gas, from a stirring, a vibration, etc. that penetrate intothe supercooled zone and into the pasty zone and modify the direction ofthe convection movements.

This is a continuous macrosegregation, this term designating the factthat the macrosegregation takes place continuously over all or a portionof the height of the ingot, in other terms it is substantially uniformaccording to the casting axis.

The phenomenon of intermittent macrosegregation has not been describedas much in literature and results in the formation of V-shaped strips oneither side of the negative central macrosegregation. These V-shapedstrips are alternatively enriched and depleted with eutectic andperitectic alloy elements. These strips can be observed by carrying outX-ray radiographies of vertical segments of ingots, typically in theplane L/TC at mid-width, when the segregated elements absorb the X-raysin a differentiated manner from the atoms of the metal comprising theingot. Other means make it possible to view this phenomenon, for examplethe echography or the observation with the naked eye of anodizedvertical segments, due to the difference in optical reflectivity betweenthe enriched or depleted zones with alloy elements. Generally, theintermittent macrosegregation is the most marked on the region T/2.5 ofthe thickness, the region T/2 corresponding to the central axis of theingot. According to a nomenclature known to those skilled in the art,the term T/n, where n is a positive number, designates a region locatedat a distance T/n of an edge of the ingot, where T designates athickness of the ingot.

The periodic intermittent macrosegregations appear very early after thestarting of the casting, as soon as an inclined front is formed betweena solid zone and a liquid zone. They are observed in all cases ofcasting of aluminum alloys loaded with aluminum alloys, cast typicallyaccording to formats with a thickness greater than 300 mm, thisthickness threshold depending itself of the casting speed.

The publication R. C. Dorward et al. “Banded segregation patterns in DCcast AlZnMgCu alloy ingots and their effect on plate properties”Aluminum, 1996, 72. Jahrgang, 4, p. 251-259 describes the formation ofstrips of intermittent segregations in an alloy 7000. According to theseauthors, this phenomenon is due to avalanches of grains triggeredperiodically by convective oscillations of the sump, i.e. the liquidphase of the metal, in liaison with a mechanism for the emission ofswirls. This article shows in particular that the intermittentmacrosegregation can be at the origin of variations in mechanicalproperties, for example in the tenacity, on the sheets obtained fromcrude casting products. It is therefore advantageous to find a castingmethod that would suppress these intermittent macrosegregations.

The reduction or the suppression of continuous macrosegregations, forexample the central macrosegregation, has already been described. Inparticular it has been shown that the application of a magnetic field,for the purposes of stirring or braking of the flows, made it possibleto limit the appearance of macrosegregations continues. DocumentUS5375647 describes for example a method for reducing centralmacrosegregation occurring during the casting of a metal alloy ingot.This method comprises the application, during the cooling, of a staticmagnetic field generated by at least one coil passed through by a directcurrent.

Document FR2530510 describes a method for the electromagnetic casting ofmetals wherein a stationary magnetic field and a variable frequencymagnetic field are made to act simultaneously, in order to produce bothradial vibrations within the metal that is not yet solidified, and limitthe stirring.

B. Zhang et al “Effect of low-frequency magnetic field onmacrosegregation of continuous casting aluminum alloys” MaterialsLetters 57 (2003) pp. 1707-1711 applied a variable magnetic field at lowfrequency (between 10 and 100 Hz) to a billet of 200 mm made of alloyAA7075 and observed a beneficial effect on the decrease in the centralmacrosegregation, mainly for a frequency of 30 Hz.

EP 2682201 describes a method of electromagnetic stirring using twoinducers mounted symmetrically with respect to the other in relation tothe vertical plane of symmetry of an ingot mold. These inducers generatetwo electromagnetic fields of different frequencies propagatingaccording to opposite directions along a vertical axis. At least one ofthe inducers generates a magnetic field at a resonance frequency of theliquid metal.

WO 2014/155357 relates to methods and an apparatus intended to displacea melted metal, the electromagnetic inducer comprising at least twopairs of electromagnetic poles and a first magnetic field componentbeing generated between a pole in a first pair of electromagnetic polesand a second pole in a different pair of electromagnetic poles, and asecond magnetic field component being generated between the two poles inone or several pairs of electromagnetic poles, the second magnetic fieldcomponent as such generating one or several eddy currents in the meltedmetal.

The inventors have considered that the methods described hereinabove donot make it possible to effectively reduce the appearance ofintermittent macrosegregations. They propose a method that makes itpossible to limit the formation of such macrosegregations, and eveneliminate them, so as to better control the mechanical properties of theproducts coming from the casting.

DESCRIPTION OF THE INVENTION

An object of the invention is a method for casting an aluminum alloyingot in a substantially rectangular ingot mold comprising the followingsteps:

preparing the aluminum alloy;

casting the aluminum alloy in the ingot mold, along a vertical axis offlow, the alloy being cooled, during the casting, by a runoff of acoolant in contact with the solidified metal;

during the casting, application of a magnetic field of which theamplitude is periodically varied according to a frequency, said magneticfield being generated by at least one magnetic field generator arrangedat the periphery of the ingot mold, in such a way as to apply a Lorentzforce at different points of a liquid portion of the alloy in theprocess of solidification;

the magnetic field applied being a traveling magnetic field, propagatingalong an axis of propagation, in such a way that a maximum amplitude ofthe magnetic field propagates along said axis of propagation, defining apropagation wavelength, said traveling magnetic field driving apropagation, along said axis of propagation, a Lorentz force of maximumintensity; the method being characterized in that a magnetic parameterreferred to as a force parameter, governing a Lorentz force value ofmaximum intensity, is variable in a predetermined time interval, saidparameter being:

said maximum amplitude of the magnetic field;

and/or said frequency of the magnetic field;

and/or the propagation wavelength of the magnetic field; in such a wayas to obtain a modulation, in said time interval, of said Lorentz forceof maximum intensity propagating along the axis of propagation.

The method can comprise any of the following characteristics, takenindividually or in combination:

the section of the ingot mold, in a horizontal plane, defines athickness and a length, the thickness being less than or equal to thelength, the thickness being greater than 300 mm and preferably at least400 mm;

the frequency of the magnetic field is less than 5 Hz, or 2 Hz or 1 Hz;

the Lorentz force of maximum intensity, propagating along the axis ofpropagation, varies by at least 30 N.m⁻³ in a time interval between 20seconds and 10 minutes;

the magnetic field is such that the absolute value of the variation ofthe density of the maximum Lorentz force is greater than or equal to0.05 N.m⁻³.s⁻¹ during said time interval;

the axis of propagation of the maximum amplitude of the magnetic fieldbelongs to a plane parallel to the direction of casting;

during the casting, the variation in the force parameter is periodical,the period being between 20 s and 20 minutes, or between 1 minute and 15minutes, or between 2 minutes and 10 minutes;

during the casting, the Lorentz force of maximum intensity is not equalto zero.

during the casting, the variation in the force parameter is not obtainedvia a periodic interruption in the travelling field.

the dimensionless Hartmann number, at at least one point of the liquidportion of the alloy, varies at least by a factor of 3, even by a factorof 5, in said time interval;

the aluminum alloy is chosen from alloys of types 2XXX, 6XXX or 7XXX,the thickness being at least 400 mm or 450 mm.

According to an embodiment, the generators are electromagnetic inducers,each electromagnetic inducer having a current flowing through itreferred to as induction current. The method comprises, during said timeinterval:

a variation in the intensity of the induction current;

and/or a variation of a frequency of the induction current;

and/or a variation of a distance between an electromagnetic inducer andthe ingot mold.

According to this embodiment, the method can comprise a variation in theintensity or in the frequency du induction current flowing through aninducer, the method then comprising:

a prior step of defining at least one critical value of the intensityand of the frequency of the induction current generating, on a freesurface of the aluminum alloy flowing in the ingot mold, a resonantwave;

a determination of a range of variation in the intensity or in thefrequency of the induction current according to said critical valuedefined beforehand.

The method can comprise a definition of a plurality of critical valuesof the intensity and of the frequency of the induction current, in sucha way as to define a resonance curve, representing the critical valuesof the intensity and of the frequency generating a resonance of saidfree surface, the method comprising a determination of a range ofvariation in the intensity or in the frequency of the induction currentin a range delimited by said resonance curve.

Preferably, the method comprises a variation in the frequency of theinduction current flowing through an inducer.

According to an embodiment, at least one generator is a permanentmagnet, the method comprising:

a variation in a distance between the permanent magnet and the ingotmold;

and/or a rotation of the permanent magnet, and a variation in therotation speed of the magnet;

and/or a rotation of two permanent magnets.

Another object of the invention is an aluminum alloy ingot, obtained bythe method such as described hereinabove and in the followingdescription.

The ingot can have, for an element of the alloy, of which the content byweight is greater than 0.5%, or the sum of two elements of the alloy ofwhich the individual content is greater than 0.5%, a dispersioncriterion less than 3.3, preferably less than 3, more advantageouslyless than 2.5, even more advantageously less than 2 and preferably lessthan 1.5, said dispersion criterion being defined according to thefollowing expressions:

ε=ΔC_(ZA)/ΔC_(ZR)(6) ΔC_(ZA)=max (C_(ZA))−min (C_(ZA)) (4), ΔC_(ZR)=max(C_(ZR))−min (C_(ZR)) (5),

where:

max (C_(ZA)) and min (C_(ZA)) respectively designate the maximum andminimum concentrations of the element considered or of the sum of thetwo elements considered measured in a zone of analysis, havingintermittent macrosegregations, for example between T/2.3 and T/3.3;

max (C_(ZR)) and min (C_(ZR)) respectively designate the maximum andminimum concentrations of the element considered or of the sum of thetwo elements considered in a reference zone considered as littleaffected by the intermittent macrosegregations, for example between T/6and T/12;

said concentrations being measured on at least one profile establishedat mid-width in a vertical plane L/TC and according to the direction TC,said profile being representative of said intermittent macrosegregationsaccording to said direction TC.

The ingot can have a spectral intensity criterion less than 0.01,preferably less than 0.007 and preferably less than 0.005, said spectralintensity criterion being calculated by:

determining a maximum amplitude of a Fourier transform of a profilerepresentative of an intermittent macrosegregation of an element ofwhich the content by weight is greater than 0.5% or the sum of twoelements of the alloy of which the individual content is greater than0.5%, the profile being established according to said direction TC, saidmaximum amplitude being determined in a range of spatial periods between8 and 25 mm,

standardizing said maximum amplitude by a nominal concentration C₀ ofsaid element or by the sum of the nominal concentrations of the twoelements considered.

Other advantages and characteristics shall come more clearly from thefollowing description of particular embodiments of the invention, givenby way of non-limiting examples, and shown in the figures listedhereinbelow.

FIGURES

FIGS. 1A to 1E show an example of the device and of the method accordingto prior art and according to the invention. FIG. 1A shows the maincomponents of the device while FIGS. 1B and 1C respectively show aspatial and time distribution of the amplitude of a traveling magneticfield according to prior art. FIGS. 1D and 1E respectively show aspatial and time distribution of the amplitude of a non-stationarytraveling magnetic field according to embodiments of the invention.

FIG. 2 shows a curve referred to as a resonance curve of the freesurface of the sump, showing values, referred to as critical values, ofthe intensity and of the frequency of an induction current at which aresonance of the free surface of the sump appears, this by implementinga method of electromagnetic stirring.

FIG. 3 is a radiograph of a vertical segment of a product obtained byimplementing a first embodiment of the method, representative of priorart, according to a first example, referred to as example 1,representative of prior art.

FIG. 4 shows an example of a profile of concentration of Zn along ahorizontal line of the vertical segment shown in FIG. 3 and the analysisand reference zones.

FIG. 5A shows the digital processing successively carried out on eachprofile obtained with a resolution of 0.1 mm. FIG. 5B shows a profileresulting from the processing carried out.

FIGS. 6A and 6B show characterization profiles of a product obtained byimplementing a method according to the example 1. FIG. 6A shows profilesof concentration of Zn along several horizontal lines of the verticalsegment shown in FIG. 3. FIG. 6B shows the profiles resulting fromdigital processing carried out.

FIG. 7 shows Fourier transforms of the profiles shown in FIG. 6B.

FIG. 8 shows a curve referred to as the resonance curve of the freesurface of the sump, obtained by implementing a method of a secondexample, referred to as example 2, according to the invention.

FIGS. 9, 10A, 1013 and 11 show a characterization of a product obtainedby implementing a method according to this second example. FIG. 9 is aradiograph of a vertical segment of the product. FIG. 10A shows profilesof concentration of Zn along several horizontal lines of the verticalsegment shown in FIG. 9. FIG. 10B shows the profiles resulting from thedigital processing carried out on the profiles shown in FIG. 9. FIG. 11shows Fourier transforms of these various profiles.

DETAILED DESCRIPTION OF THE INVENTION

Unless mentioned otherwise, all of the indications concerning thechemical composition of the alloys are expressed as a percentage byweight based on the total weight of the alloy. The expression 1.4 Cumeans that the content in copper expressed as a % by weight ismultiplied by 1.4. The designation of the alloys is done in compliancewith the regulations of The Aluminum Association, known to those skilledin the art.

FIG. 1A shows an example of the casting method known from prior art. Inthis example, an aluminum alloy 1 flows in an ingot mold 2, through anopening 2 i. The ingot mold 2 extends according to a vertical axis Z. Itis delimited by a peripheral chamber of which the section, in ahorizontal plane XY, is parallelepipedic. A coolant 3, for examplewater, flows against the wall of the solidified product. This method isknown as semi-continuous direct chill casting. A false-bottom 4 can betranslated in such a way as to move away from the opening 2 i during thecasting. The ingot mold 2 extends, parallel to a first horizontal axisX, according to a thickness e and, parallel to a second horizontal axisY, perpendicular to the X axis, according to a length l. The thickness eis for example greater than 300 mm. It is beyond such a thickness thatthe intermittent macrosegregations 11 appear in a marked way. Under theeffect of the cooling, a solid zone 1 s is formed, in the vicinity ofthe cooled chamber, around a liquid zone 1 l, designated by the term“sump”. The interface between the liquid zone 1 l and the solid zone 1 sis a front 10, with the latter progressing towards the center of theingot mold as the solidification of the alloy takes place. At the end ofthe cooling, a parallelepiped ingot, also designated by the term“product”, is formed.

The alloy is an aluminum alloy of the series 1XXX, 2XXX, 3XXX, 4XXX,5XXX, 6XXX, 7XXX or 8XXX. The alloys of which the mass fraction in alloyelements is greater than 1%, even greater than 3% or even 5% areparticularly suited to a method according to the invention, because thegreater this mass fraction of these alloy elements is, the more markedthe intermittent macrosegregations are. The invention is particularlyadvantageous for products of alloy 2XXX, 5XXX, 6XXX or 7XXX of which thethickness is at least equal to 400 mm even 450 mm.

A magnetic field generator 5 is shown, able to generate a magnetic fieldB intended to be applied to the liquid zone 1 l of the alloy. Such agenerator can be a permanent magnet or an electromagnetic inducer, thelatter generating a magnetic field when it is passed through by anelectric current, referred to as induction current.

The magnetic field B applied to the liquid zone 1 l is an alternatingfield, of amplitude B₀ and of frequency f. The effect of this magneticchamber is to apply a stirring of the sump, under the effect of Lorentzforces that are applied on the metal liquid zone 1 l. Indeed, theapplication of a magnetic field B generates, in the alloy, the formationof a resulting electric current J, within the liquid zone of the alloysubjected to the magnetic field, in the appearance of a Lorentz force Fsuch that F ∝J×B where × designates the vector product operator, and ∝designates a proportional relation. This Lorentz force has a componentoscillating at a frequency double the frequency ƒ of the magnetic field.

Due to the thickness of the ingot mold, the frequency ƒ is chosen insuch a way as to allow for a sufficient penetration of the magneticfield B in the sump, in such a way as to obtain an effective stirring ofthe liquid. The frequency ƒ is as low as the thickness of product ishigh. In the case of an aluminum alloy with a thickness greater than 300mm, the frequency is more preferably less than 5 Hz, and even moreadvantageously less than 2 Hz or 1 Hz.

The generator 5 is able to generate a traveling magnetic field. The termtraveling magnetic field designates an alternating magnetic field, ofwhich the amplitude B₀ is not constant, and varies between a minimumvalue and a maximum amplitude B₀ ^(max), the maximum amplitude B₀ ^(max)propagating along an axis of propagation Δ, more preferably straight.The term amplitude means the maximum value that a periodic magnitudehas. More preferably, the axis of propagation belongs to a planeparallel to the direction of casting.

The distance λ separating two maximas of amplitude of the magnetic fieldis the wavelength of the traveling magnetic field. FIG. 1B shows anexample of the distribution of the amplitude B₀ of un traveling magneticfield along an axis of propagation Δ at an instant t (continuous line),and at an instant t+Δt (dotted line). On the axis of propagation, acoordinate r is shown corresponding to the position of a point of thesump. FIG. 1C shows a time change of a traveling alternating magneticfield at this point. Due to the propagation of the value of the maximumamplitude B₀ ^(max), the amplitude of the magnetic field, at this point,varies between a minimum value B₀ ^(min) and the value B₀ ^(max) thelatter not changing over time.

A traveling magnetic field generator 5 can be formed by severalelectromagnetic inducers arranged around the peripheral chamber. TheLorentz force, at a coordinate point r of the sump, comprises anoscillating component, modulated according to a frequency 2ƒ double thefrequency of the magnetic field. The amplitude F₀ of the density of theoscillating Lorentz force can be expressed according to the expression:

F₀(r)=½σƒλB₀ ²(r) (1), where σ designates the electrical conductivity.

It is possible to define a travelling speed V_(G) of the magnetic fieldV_(G)=ƒλ(2) in which case the expression (1) can be expressed asfollows:

F₀(r)=½σV_(G)B₀ ²(r) (3)

As such, the amplitude of the Lorentz force, at a point r of the sumpdepends on the square of the amplitude of the magnetic field applied atthis point. The application of a traveling magnetic field results, at apoint of the sump, in a modulation of its amplitude. As such, theamplitude of the magnetic field at a point of the sump varies as afunction of time, between a minimum amplitude B₀ ^(min) and a maximumamplitude B₀ ^(max). The same applies to the Lorentz force density, thelatter having, at a point r of the sump, a maximum value when theamplitude of the magnetic field, at this point, is maximal. In thecoordinate system XYZ, linked to the ingot mold 2, the propagation of amaximum value of the amplitude of the magnetic field B₀ ^(max), along anaxis of propagation, drives, simultaneously, the propagation of aLorentz force of maximum intensity F_(max) according to the axis ofpropagation Δ. The combination of the forces propagating along the axisof propagation establishes a movement of the liquid according to thisaxis forming an electromagnetic pump element.

The inventors have observed that by modulating, over time, the maximumamplitude of the Lorentz force F_(max) propagating in the sump, theintermittent macrosegregations are attenuated, and even disappear, andthis particularly on ingots of which the thickness is greater than 300mm.

This time modulation can be obtained by a variation in a parameter,referred to as the magnetic force parameter, controlling the amplitudeof the Lorentz force density explained in the equations (1) and (3), forexample:

the value of the maximum amplitude B₀ ^(max) of the magnetic field;

de the frequency f of the magnetic field;

the wavelength λ of the traveling magnetic field.

When the traveling magnetic field is generated by a plurality ofelectromagnetic inducers arranged at the periphery of the ingot mold,the time modulation of the Lorentz force density can be obtained bymodifying the pole pitch, i.e. the out of phase between the inductioncurrents flowing in each inducer. Such a modification makes it possibleto vary the wavelength λ of the traveling magnetic field, i.e. thedistance between two maximas propagating along the axis of propagation.The frequency of the induction current flowing in the inductors can bevariable, which modifies the frequency ƒ of the magnetic field. Theamplitude of the induction current can also be variable, which modifiesthe value of the maximum amplitude B₀ ^(max) of the magnetic field. FIG.1D shows an embodiment wherein the value of the maximum amplitude B₀^(max) of the magnetic field and the wavelength λ of the travelingmagnetic field are variable over time. As such, a spatial distributionof the amplitude B₀(t) in the sump is shown, at an instant t (continuousline), as well as a spatial distribution of the amplitude B₀(t+Δt), atan instant t+Δt (dotted line). During the time interval Δt, the maximumamplitude B₀ ^(max) varies between B₀ ^(max)(t) and B₀ ^(max)(t+Δt).Likewise, the wavelength λ was modified, passing from λ(t) to λ(t+Δt).In FIG. 1E, which shows a time change of a traveling alternatingmagnetic field at a point, an embodiment is shown wherein the value ofthe maximum amplitude B₀ ^(max) of the magnetic field varies, over time,for a frequency ƒ and a constant wavelength λ.

Therefore, in the examples shown in FIGS. 1D and 1E, the maximumamplitude of the Lorentz force, propagating in the sump, varies betweent and t+Δt, between the values F_(max)(t) and F_(max)(t+Δt).

The time modulation of a force parameter is implemented during thecasting, for a significant duration, more preferably greater than 50%even 80% of the duration of the casting. This time modulation can forexample be applied for at least 30 minutes, even at least 1 hour.

A traveling magnetic field B can in particular be generated from twoinducers arranged on the same face of the ingot. The inducers arearranged more preferably facing a large face of the ingot, i.e. one ofthe two faces of the ingot having the largest vertical section. Theinducers can be superimposed upon each other, in such a way as togenerate a so-called vertical out of phase, or arranged side by side, insuch a way as to generate a so-called horizontal out of phase. In theexamples described hereinafter, a device was used described inapplication WO2014/155357, and more precisely according to theconfiguration described in liaison with FIGS. 19 and 20A, wherein threeinducers, oriented according to the vertical axis Z, are arranged facingeach large face of the ingot.

The traveling magnetic field can also be generated from one or severalpermanent magnets arranged at the periphery of the ingot mold and setinto motion in relation to the latter. For example, it is possible togenerate a traveling magnetic field by rotating a permanent magnet.

A variation in the parameters of the traveling magnetic field, whetherconcerning its amplitude, its frequency or its wavelength makes itpossible to apply a non-stationary Lorentz force in the sump. Theinventors have observed that this makes it possible to attenuate theappearance of intermittent macrosegregations and even make themdisappear. Such conditions probably influence the recirculations thatare spontaneously produced in the sumps, and reduce the consequencesthereof.

Preferably, in the sump, the speed of the variation in the maximumLorentz force density is greater than 0.05 N.m⁻³.s⁻¹, and morepreferably greater than 0.1 N.m⁻³.s⁻¹, and more preferably greater than0.2 N.m⁻³.s⁻¹. In an embodiment the maximum speed of the variation inthe maximum Lorentz force density during the casting is at least 1N.m⁻³.s⁻¹ and more preferably at least 2 N.m⁻³.s⁻¹.

More preferably, the variation in one or several force parameters takesplace in a time interval less than or equal to the characteristicdurations of recirculations generated by natural convection. Thesedurations vary according to the thickness of the ingot and the castingspeed. Considering thicknesses e between 300 mm and 700 mm, and castingspeeds between 30 mm/min and 80 mm/min, the characteristic durations ofthe recirculations extend between 20 seconds (thickness of 300 mm,casting speed of 30 mm/min) and 10 minutes (thickness of 700 mm, castingspeed of 80 mm/min). As such, the force parameters vary in a timeinterval Δt determined according to these characteristic durations. Theterm variation means a significant variation, of at least 10% in theforce parameter considered, and preferably at least 20% and even 30% ofthe force parameter.

The variation of a force parameter can be periodical, the time period ofvariation able to be about a characteristic duration of recirculation,i.e. be between 20 seconds and 10 minutes according to the dimension andspeed conditions of the casting. Preferably, in the sump, during thetime period of variation, the maximum density of the Lorentz forcevaries by at least 30 N.m⁻³, and advantageously by at least 40 N.m⁻³,and preferably at least 50 N.m⁻³, and even more preferably by at least60 N.m⁻³.

The variation in a force parameter can also be monotonous during thecasting, for example according to an increasing or decreasing functionbetween the starting and the ending of the casting, the value of theforce parameter varying continuously or in successive increments.

Advantageously, during the casting, the Lorentz force of maximumintensity is not equal to zero. Typically, it is equal to zero when thecurrent in the inducers or the coils is equal to zero. Thereforeadvantageously, the variation in the force parameter is not obtained viaa periodic interruption in the travelling field.

Advantageously, during the casting, the Lorentz force of maximumintensity is greater than 80 N/m³, more preferably greater than 100N/m³, more preferably greater than 120 N/m³, even more preferablygreater than 140 N/m³. The inventors indeed observed that thesuppression of the intermittent macrosegregations was not optimal whenthe force was too weak was shown in the example 5 (FIG. 20a to d ). Theminimum value starting from which the suppression of the intermittentmacrosegregations is improved depends on all of the casting parameters,in particular the stirring method, the position of the inducers inrelation to the plate and the composition of the alloy.

According to an embodiment, the frequency ƒ and/or the maximum amplitudeB₀ ^(max) of the magnetic field are modified respectively by varying thefrequency and the amplitude of the induction current flowing ininducers. For this, the method can include a prior step of defining anoperating range, i.e. a range in the variation of the frequency and/orof the intensity of the induction current. This prior step comprises thedetermination of one or of several values of frequency/intensity pairs,referred to as critical values, generating, at the free surface 1 _(sup)of the sump, a resonance, the resonance resulting in the appearance ofsignificant oscillations of said free surface 1 _(sup), the latter beingshown in FIG. 1A. These significant oscillations are generally observedwith the naked eye. The term significant oscillation means for examplean oscillation of which the amplitude is greater than or equal to 5 mmaccording to the vertical axis Z. For example, the frequency of thecurrent is fixed and the intensity of the induction current is increaseduntil a significant oscillation is observed.

By considering different critical values of frequency (or of intensity),it is possible to experimentally determine a resonance curve R, in afrequency/intensity plane that corresponds to the various pairs(frequency/intensity) at which a resonance is observed at the freesurface of the sump. Using this curve R, a range in the variation of theintensity and/or of the frequency is determined, in such a way as toprevent or limit the appearance of a resonance of the free surface ofthe sump. Indeed, the resonance curve delimits a zone of stability and azone of instability, wherein the casting can become dangerous. However,modulating the frequency or the intensity of the induction current, andtherefore the frequency f or the maximum amplitude B₀ ^(max) of thetraveling magnetic field, makes it possible to temporarily approach theresonance curve R, for example periodically, while still remaining inthe zone of stability. This makes it possible to maximize the intensityof the Lorentz force, and therefore the stirring of the sump, whilestill remaining in acceptable safety configurations. Indeed, in thevicinity of the resonance curve, the stirring effect is particularlyimportant.

Such a resonance curve R depends on the casting conditions, i.e.dimensions of the ingot mold, the casting speed, the configuration ofthe magnetic field applied, the latter depending on the magnetic fieldgenerator, i.e. on the inducers or on the permanent magnet or magnetsused. A resonance curve R is shown in FIG. 2, this curve having beenobtained by casting an ingot of thickness 525 mm×1650 mm, according to acasting speed of 45 mm/min, a magnetic stirring being carried out by theapplication of a magnetic field by three inducers arranged in front ofeach large face of the ingot and out of phase by 90° in order to form ahorizontal electromagnetic pump element, as mentioned hereinabove. Thisfigure also shows plots representing a percentage of the intensity of aLorentz force, referred to as the nominal force, 100% corresponding tothe maximum intensity of the induction current that can be used in theinstallation when the frequency is equal to 0.2 Hz. This intensitycorresponds to the appearance of a resonance at the frequency of 0.2 Hz.Preferably, the intensity and the frequency of the induction current islocated in a space delimited by the curve representing a certainpercentage of the intensity of the nominal Lorentz force, for example10% of this intensity, and the resonance curve.

Preferably, the method comprises a variation in the frequency of theinduction current flowing through an inducer. The inventors found thatit was advantageous to vary the frequency because the variation in thepenetration in the field that results therefrom makes it possible tomore effectively vary the force gradient in the thickness and the depthof the liquid well. Moreover, the power electronics make the variationin frequency faster than the variation in intensity; which gives anadditional degree of freedom towards weaker periods of unsteady forcing.It is indeed advantageous to decouple the characteristic hydrodynamictimes from the characteristic times of the solidification in order toprevent intermittent macrosegregations.

Another example of a curve is shown in FIG. 8 and will be commented onlater in liaison with the examples. FIGS. 2 and 8 show the resonancecurve R, determined experimentally, as well as the curve representing aLorentz force of which the intensity is equal to 10% of the nominalLorentz force defined beforehand.

The variation in one or several force parameters can in particular makeit possible to alternate periods during which the dimensionless Hartmannnumber Ha is respectively low, typically less than 1, and high,typically greater than 3, and even 5. The dimensionless Hartmann numberHa is a number that is commonly used in the field ofmagnetohydrodynamics. It represents a ratio between the magneticviscosity and the viscosity of a loaded liquid flowing in a magneticfield. The higher this number is, the higher the contribution of theLorentz forces is. More preferably the dimensionless Hartmann number Haalternates with a ratio between low and high values by at least 3 or byat least 5. Such a configuration is preferred, because it makes itpossible to alternate periods during which the kinetic energy applied bythe magnetic field opposes the natural convection of the liquid metal,and periods during which the natural convection predominates.

As described in liaison with the examples presented hereinafter, theproducts obtained by a method according to the invention have a limitedintermittent macrosegregation in relation to methods of prior art, andeven imperceptible. In the examples that follow, the characterization ofthe products was carried out by analyzing horizontal profiles (accordingto the axis TC) of a radiograph carried out at mid-width according to avertical plane L/TC, these profiles being calibrated in order to obtainthe spatial distribution of elements of heavy alloys of the type Zn orCu. The zones enriched with such heavy elements, more absorbent, appearin the form of dark spots on the negative of the radiographs carried outand therefore light spots on the radiographs presented. An example ofobtaining the profile of the concentration in Zn using a radiograph ofan alloy Al—Zn is shown in FIG. 4.

The terms L, TL and TC, known to those skilled in the art, correspondentrespectively to the dimension of the ingot according to the verticalaxis, the axis referred to as “long cross” and according to the axisreferred to as “short cross”.

In a complementary or alternative manner, it is possible to conductchemical analyses according to horizontal profiles, in such a way as toquantify the spatial distribution of said chemical elements according tothe axis TC. An intermittent macrosegregation can be characterized by amaximum difference in mass of an alloy element, here Zn, in the zonethat is the most marked by the intermittent macrosegregations, i.e. inthe vicinity of T/2.5.

In order to quantify the intermittent macrosegregation, theconcentration profiles, obtained by radiography or par any other method,with a spatial resolution of 0.1 mm were processed as shown in FIG. 5A.The profile obtained with the resolution of 0.1 mm is the crude profilereferenced as profile A. A sliding average over 2 mm makes it possibleto overcome the microsegregation, the smoothed profile obtained isreferenced as profile B. Another sliding average o the crude profileover 50 mm makes it possible to overcome intermittent macrosegregations,and obtain the continuous macrosegregation profile, the profile obtainedbeing a profile referred to as a basic profile, referenced as profile C.The profile C is subtracted from the profile B in order to obtain aprofile referred to as corrected, corresponding to the intermittentmacrosegregation, the corrected profile being referenced as profile D.Such a profile is shown in FIG. 5B. As can be seen in this FIG. 5B, thecorrected profile is mainly representative of the intermittentmacrosegregation, and is not or is little affected by the centralcontinuous macrosegregation and by the microsegregation. Such acorrected profile makes it possible to characterize the intermittentmacrosegregation.

It is then possible to calculate a maximum difference in concentrationin a zone of analysis Z_(A) located between T/2.3 and T/3.3, thismaximum difference able to be expressed according to the followingequation:

ΔC_(ZA)=max (C_(ZA))−min (C_(ZA))  (4)

where max (C_(ZA)) and min (C_(ZA)) respectively designate the maximumand minimum concentrations of the element considered measured betweenT/2.3 and T/3.3.

The element considered is an element of which the content by weight inthe alloy is greater than or equal to 0.5%. This can be, morepreferably, the major element of the alloy, the term major elementcorresponding to the definition given by The Aluminum Association.

The maximum difference ΔC_(ZA) can be standardized by the nominalconcentration C_(O) of the element considered. The products according tothe invention preferably have a value of such a standardized ratio lessthan 10% and more preferably less than 8% or even less than 6%. Howeverthe absolute value of ΔC_(ZA) can be influenced by the thickness of theproduct, the nature of the element considered, in particular itspartition coefficient and/or its concentration. It is therefore usefulto characterize the products obtained by the method according to theinvention to calculate, as a reference, a maximum difference in a zoneof reference Z_(R) that is little sensitive to the intermittentmacrosegregations, located between T/6 and T/12, this maximum differenceable to be expressed according to the following equation:

ΔC_(ZA)=max (C_(ZR))−min (C_(ZR))  (5)

where max (C_(ZR)) and min (C_(ZR)) respectively designate the maximumand minimum concentrations of the element considered measured betweenT/6 and T/12.

A dispersion criterion E is thus obtained that makes it possible toevaluate for the element considered the intermittent macrosegregation:

ε=ΔC_(ZA)/ΔC_(ZR)  (6)

In order to overcome local variations in composition, it isadvantageous, to determine ΔC_(ZA) and ΔC_(ZR), to calculate an averageover at least 5 profiles of concentration that are separated by at least10 mm.

The lower ε is, the less marked the intermittent macrosegregations are.The products obtained by the method according to the invention morepreferably have a dispersion criterion ε less than 3.3, preferably lessthan 3, more advantageously less than 2.5, even more advantageously lessthan 2 and preferably less than 1.5.

According to a nomenclature known to those skilled in the art, T/ndesignates a distance in relation to an edge of the ingot, according toa horizontal axis, T/2 corresponding to the center of the ingot.

It is also useful to carry out an analysis via Fourier transform of thecrude profile of composition and to standardize it by the nominalcomposition of the element. Such an analysis makes it possible toidentify spatial periods that characterize the intermittentmacrosegregation. The intermittent macrosegregation has periods between8 and 25 mm. When the intermittent macrosegregation is substantial, apeak in the amplitude of the Fourier components is then observed forspatial periods between 8 and 25 mm. A dimensionless criterion ofspectral intensity ζ is determined which corresponds to the maximumamplitude of the Fourier components in a spatial period range between 8and 25 mm, standardized by the nominal concentration C_(o) of theelement considered. The products obtained by the method according to theinvention more preferably have a criterion ζ less than 0.01, preferablyless than 0.007 and preferably less than 0.005.

The dispersion criterion E and spectral intensity criterion ζ areadvantageously applied to the major element of the alloy considered,typically to the Zn for an alloy 7xxx or to the Cu for an alloy 2xxx. Itis also possible to apply these criteria to the sum of two elements, forexample the sum Zn+Cu in certain alloys 7xxx or the sum Mg+Si in thealloys 6xxx. These criteria can also be applied to an element of whichthe content by weight in the alloy is greater than or equal to 0.5% orto the sum of two elements of the alloy of which the individual contentis greater than 0.5%,

In the case where the sum of two elements is considered, the values forstandardizing the maximum difference ΔC_(ZA), and/or the Fouriertransform correspond to the sum of the nominal concentrations of theelements considered.

The ingots of rectangular section obtained by the method according tothe invention can be used as cast or after working, optionally aftersolution heat treatment and quenching and aging for age-hardenablealloys. Advantageously the ingots with a rectangular section obtained bythe method according to the invention are rolled and/or forged.

Example 1

A casting of an alloy AA7035 was carried out without electromagneticstirring. The composition of the alloy cast comprising a nominalconcentration of Zn of 5.6% by weight, a nominal concentration of Mg of1.3% by weight. The format of the ingot was 1650 mm×525 mm. This exampleis representative of prior art. The grain refining was carried out witha concentration of refining agent AlTiB 5:1 of 1 Kg/t. The casting speedwas 35 mm/min. FIG. 3 shows a radiograph of the ingot at mid-width alonga plane L/TC, whereon the negative central macrosegregation and theintermittent macrosegregations are clearly identifiable. FIG. 6A showsdifferent horizontal crude profiles of the content of Zn, along an axisTC, as well as smoothed profiles B are obtained by a sliding averageover 2 mm deduced from FIG. 3. The radiograph makes it possible toquantify that the elements at the origin of a contrast with respect tothe aluminum, namely in this case the Zn. This remark applies to thefollowing example 2. The negative central macrosegregation is clearlyobserved, maximum at T/2, the intermittent macrosegregations beingobserved between T/2.3 and T/3.3. FIG. 6B shows the different profilescorrected for the content of Zn (profiles D), along an axis TC, obtainedafter subtraction of each smoothed profile (profile B) by a basicprofile (profile C) representative of the continuous macrosegregation.

The value of the maximum differences of the content of Zn was 0.75% byweight for ΔC_(ZA) and 0.19% by weight for ≢C_(ZR), the value of themaximum standardized differences in the analysis zone and in thereference zone being as such respectively 13.3% and 3.5%. The value ofthe dispersion criterion ε such as defined by the equation (6) was 3.9.The Fourier transform of each profile was calculated, and is shown inFIG. 7, after standardization by the nominal composition of Zn: 5.6% byweight. The axis of the abscissa represents the spatial period, between0 and 30 mm. Different predominant peaks are observed, corresponding todifferent spatial periods distributed between 8 and 25 mm, and moreparticularly between 10 mm and 25 mm. The spectral intensity criterion ,which corresponds to the maximum amplitude of the Fourier componentsbetween 8 and 25 mm, standardized by the nominal concentration C_(o) ofthe Zn, was for all of the profiles at least 0.01.

Example 2

During a second example, a casting of an alloy AA7035 was carried outwith an electromagnetic stirring. The composition of the cast alloy hada nominal concentration of Zinc of 5.6% by weight and a nominalconcentration of Magnesium of 1.3% by weight. The format of the ingotwas 1650 mm×525 mm. The grain refining was carried out with aconcentration of refining agent AlTiB 5:1 de 1 Kg/t. The casting speedwas 35 mm/min. The electromagnetic stirring was obtained by setting up,opposite each face L/TL of the ingot, (corresponding to a plane YZ inthe coordinate system indicated in FIG. 1A), three inducers orientedalong the vertical axis Z, passed through by an alternating current, ofa frequency 0.25 Hz, out of phase with respect to one another by 60° andspaced apart from one another by 0.6 m, as such forming anelectromagnetic pump element. The distance between the inductors and theingot was 172 mm. The electromagnetic pump elements on each face wereoriented in the opposite direction. The inducers generated a travelingmagnetic field along a horizontal plane, the traveling axis beingparallel to the direction TL, the wavelength λ was 3.6 m. The maximumdensity of the Lorentz force induced in the liquid sump was variedbetween about 180 N/m³ and 240 N/m³with a variation speed of 2 N.m⁻³.s⁻¹by modifying the nominal value of the current in the inducers. Theresonance curve, corresponding to these casting conditions, is shown inFIG. 8. The variation in the intensity of the induction current isrepresented, in this figure by a double arrow.

FIG. 9 shows a radiography of the ingot according to a plane L/TC,whereon the negative central macrosegregation at T/2 can be identified.FIG. 10A shows different horizontal crude profiles of the content of Zn(profile A) and smoothed (profiles B), along an axis TC. The negativecentral macrosegregation is distinguished, maximum at T/2. FIG. 10Bshows the different horizontal profiles of the content of Zn, along anaxis TC, of the corrected profile type (profiles D) obtained aftersubtraction of the profile corresponding to the continuousmacrosegregation.

The value of the maximum differences of the content of Zn was 0.24% byweight for ΔC_(ZA) and 0.28% by weight for ΔC_(ZR), the value of thestandardized maximum differences in the zone of analysis and in the zoneof reference being respectively 4.3% and 5%. The value of the dispersioncriterion ε such as defined by the equation (6) was 0.9: theintermittent macrosegregation in the zone of analysis between T/2.3 andT/3.3 was removed. The Fourier transform of each profile was calculated,and is shown in FIG. 11, after standardization by the nominalcomposition of Zn: 5.6% by weight. The axis of the abscissa representsthe spatial period, between 0 and 30 mm. Predominant peaks are no longerobserved. The spectral intensity criterion ζ, which corresponds to themaximum amplitude of the Fourier components between 8 and 25 mmstandardized by the nominal concentration C_(o) of the Zn, was for allof the profiles less than 0.005.

Example 3

In this example, a casting of an alloy AA 7050 was carried out withoutelectromagnetic stirring. The composition of the alloy was 6.3% byweight of Zn, 2.2% by weight of Mg and 2.1% by weight of Cu. The formatof the ingot was 1650×525 mm. The grain refining is carried out using agrain refining rod AlTiC3:0.15 with an addition ratio of 1 kg/ton. Thecasting speed was 45 mm/min. It forms the reference of the example 4.

FIG. 12 shows a radiography of the ingot according to a plane L/TC,whereon the negative central macrosegregation at T/2 can be identified.FIG. 13a shows the smoothed horizontal profile of the sum of twoelements Zn and Cu (profiles B) along an axis TC, deduced from theradiograph of FIG. 12. Indeed, the radiograph makes it possible toquantify only the elements at the origin of a contrast in relation tothe aluminum, namely in this case the Zn and the Cu. This remark appliesto the following examples 4 and 5. FIG. 13b shows the differenthorizontal profiles of the concentration of Zn+Cu, along an axis TC, ofthe corrected profile type (Profiles D) obtained after subtraction ofthe profile corresponding to the continuous macrosegregation. The valueof the maximum differences of the sum Zn+Cu was 0.81% by weight forΔC_(ZA) and 0.19% for ΔC_(ZR). The value of the dispersion criterion εsuch as defined by the equation (6) was 4.4. FIG. 14 shows the Fouriertransform of each profile, after standardization by the sum of thenominal compositions of Zn and Cu: 8.3% by weight. The axis of theabscissa represents the spatial period, between 0 and 30 mm. Thespectral intensity criterion ζ, which corresponds to the maximumamplitude of the Fourier components between 8 and 25 mm standardized bythe sum of the nominal compositions of Zn and Cu, was for one of theprofiles greater than 0.01 or for all of the profiles greater than0.007.

Example 4

In this example, a casting of an alloy AA 7050 was carried out. Thecomposition of the alloy was 6.3% by weight of Zn, 2.2% by weight of Mgand 2.1% by weight of Cu. The section of the ingot was 1650×525 mm. Thegrain refining is carried out using a grain refining rod AlTiC3:0.15with an addition ratio of 1 kg/ton. The casting speed was 45 mm/min. Theelectromagnetic stirring was obtained by setting up, opposite each faceL/TL of the ingot, (corresponding to a plane YZ in the coordinate systemindicated in FIG. 1A) three coils oriented according to the axis z andpassed through by an alternating current which was out of phase, in thecentral coil, by 90° in relation to the current in the end coils. Thewavelength of the traveling field was 2.4 m. The electromagnetic pumpelements obtained as such were arranged as a mirror in relation to eachface L/TL of the ingot, the traveling direction being parallel to thelong-cross direction, the traveling generated diverging from themid-width of the ingot. The unsteady forcing was obtained by theimposing of a cyclical variation in the frequency of the alternatingelectric current that passed through the coils, such as shown by thedouble arrow in the frequency vs. intensity diagram of FIG. 15. Themaximum density of the Lorentz force generated as such by the variationin the frequency between 0.450 and 0.600 Hz was varied between about 110N/m³ and 150 N/m³ over a period of 3 min which corresponds to a speed ofvariation of about 0.22 N/m³/s.

FIG. 16 shows a radiograph of the ingot according to a plane L/TC,whereon the negative central macrosegregation at T/2 can be identified.The intermittent macrosegregations are greatly attenuated in relation tothe reference (FIG. 12), as shown in FIGS. 17a and 17 b.

FIG. 17a shows the smoothed horizontal profile of the sum of theelements of Zn+Cu (profiles B) along an axis TC, deduced from theradiograph of FIG. 16. FIG. 17b shows the different horizontal profilesof the sum of the two elements Zn and Cu, along an axis TC, of thecorrected profile type (Profiles D) obtained after subtraction of theprofile corresponding to the continuous macrosegregation. The value ofthe maximum differences of the content of Zn+Cu was 0.30% by weight forΔC_(ZA) and 0.14% for ΔC_(ZR). The value of the dispersion criterion εsuch as defined by the equation (6) was 2.2. The intermittentmacrosegregation in the zone of analysis was therefore reduced and isshown in FIG. 18, after standardization by the sum of the nominalcompositions of Zn and Cu: 8.3% by weight. The axis of the abscissarepresents the spatial period, between 0 and 30 mm. The spectralintensity criterion which corresponds to the maximum amplitude of theFourier components between 8 and 25 mm standardized by the sum of thenominal compositions of Zn and Cu, was for all of the profiles less than0.005.

Example 5

In this example, a casting of an alloy AA7050 was carried out. Thecomposition of the alloy was 6.3% by weight of Zn, 2.2% by weight of Mgand 2.1% by weight of Cu, the contents of the other elements were allless than 0.5% by weight. The section of the ingot was 1650×525 mm. Thegrain refining is carried out using a grain refining rod AlTiC3:0.15with an addition ratio of 1 kg/ton. The casting speed was 45 mm/min. Theelectromagnetic stirring was obtained by setting up, opposite each faceL/TL of the ingot, (corresponding to a plane YZ in the coordinate systemindicated in FIG. 1A) three coils oriented according to the axis z andpassed through by an alternating current which was out of phase, in thecentral coil, by 90° in relation to the current in the end coils. Thewavelength of the traveling field was 2.4 m. The electromagnetic pumpelements obtained as such were arranged as a mirror in relation to eachface L/TL of the ingot, the traveling direction being parallel to thelong-cross direction, the traveling generated diverging from themid-width of the ingot.

The unsteady forcing was obtained by the imposition of a variationstarting from zero of the intensity of the alternating electric currentthat flowed through the coils, such as shown by the arrows in thefrequency vs. intensity diagram of FIG. 19. The intensity of the maximumvolume Lorentz force generated as such by the variation in the intensityvaried typically from 0 N/m³ to 140 N/m³ in 4 min which corresponds to aspeed of variation of 0.58 N/m³/s. In what follows, the intensity of themaximum volume Lorentz force was made to vary between 140 N/m³ and 360N/m³ in 5 min which corresponds to a speed of variation of 0.73 N/m³/s.

The results obtained are illustrated by the two radiographed verticalsegments shown in FIG. 20a (variation in the intensity between 0 N/m³ to140 N/m³ in 4 min) and FIG. 21a (variation of the force between 140 N/m³to 360 N/m³ in 5 min) which are in continuity with one another.

FIG. 20b shows the smoothed horizontal profile of the sum of the majorelements Zn+Cu (profiles B) along an axis TC, deduced from theradiograph of FIG. 20 a. FIG. 20c shows the different horizontalprofiles of the sum of the elements Zn+Cu, along an axis TC, of thecorrected profile type (Profiles D) obtained after subtraction of theprofile corresponding to the continuous macrosegregation. The value ofthe maximum differences of the content of Zn+Cu was 0.70% by weight for≢C_(ZA) and 0.22% for ΔC_(ZR). The value of the dispersion criterion εsuch as defined by the equation (6) was 3.2. FIG. 20d shows the Fouriertransform of each profile, after standardization by the sum of thenominal compositions of Zn and Cu: 8.3% by weight. The axis of theabscissa represents the spatial period, between 0 and 30 mm. Thespectral intensity criterion ζ, which corresponds to the maximumamplitude of the Fourier components between 8 and 25 mm standardized bythe sum of the nominal compositions of Zn and Cu, was for all of theprofiles less than 0.01. Note however that the spectral intensitycriterion ζ shows values greater than 0.005.

FIG. 21b shows the smoothed horizontal profile of the sum of the majorelements Zn+Cu (profiles B) along an axis TC, deduced from theradiograph of FIG. 21 a. FIG. 21c shows the different horizontalprofiles of the sum of the major elements Zn+Cu, along an axis TC, ofthe corrected profile type (Profiles D) obtained after subtraction ofthe profile corresponding to the continuous macrosegregation. The valueof the maximum differences of the content of Zn+Cu was 0.37% by weightfor ΔC_(ZA) and 0.15% for ΔC_(ZR). The value of the dispersion criterionε such as defined by the equation (6) was 2.4. FIG. 21d shows theFourier transform of each profile, after standardization by the sum ofthe nominal compositions of Zn and Cu:8.3% by weight. The axis of theabscissa represents the spatial period, between 0 and 30 mm. Thespectral intensity criterion ζ, which corresponds to the maximumamplitude of the Fourier components between 8 and 25 mm standardized bythe sum of the nominal compositions of Zn and Cu, was for all of theprofiles less than 0.005.

It is as such observed that the suppression of the intermittentmacrosegregations is improved if the force is greater than 140 N/m³.Indeed, when the force is too weak, it is observed that the value of thedispersion criterion ε

of spectral intensity ζ are greater than the preferred values of theinvention. The inventors suppose as such that an unsteady forcing thatwould consist in periodically interrupting the traveling field would notmake it possible to advantageously suppress the intermittentmacrosegregations.

1. A method for casting an aluminum alloy ingot in a substantiallyrectangular ingot mold comprising the following steps: preparing thealuminum alloy; casting the aluminum alloy in the ingot mold, along avertical casting axis, the alloy being cooled, during the casting, by arunoff of a coolant in contact with the solidified metal; during thecasting, applying a magnetic field of which the amplitude (B₀) isperiodically varied according to a frequency (ƒ), said magnetic fieldbeing generated by at least one magnetic field generator arranged at theperiphery of the ingot mold, in such a way as to apply a Lorentz force(F) at different points of a liquid portion of the alloy in the processof solidification; the magnetic field applied being a traveling magneticfield, propagating along an axis of propagation, in such a way that amaximum amplitude (B₀ ^(max)) of the magnetic field propagates alongsaid axis of propagation, defining a propagation wavelength (λ), saidtraveling magnetic field driving a propagation, along said axis ofpropagation, a Lorentz force of maximum intensity; wherein the forceparameter, governing the Lorentz force of the maximum intensity, isvariable in a predetermined time interval, said parameter being: saidmaximum amplitude of the magnetic field; and/or said frequency of themagnetic field; and/or the propagation wavelength of the magnetic field;in such a way as to obtain a modulation, in said time interval, of saidLorentz force of the maximum intensity propagating along the axis ofpropagation.
 2. The method according to claim 1, wherein the section ofthe ingot mold, in a horizontal plane, defines a thickness and a length,the thickness being less than or equal to the length, the thicknessbeing greater than 300 mm and preferably at least 400 mm.
 3. The methodaccording to claim 1, wherein the frequency of the magnetic field isless than 5 Hz, or 2 Hz or 1 Hz.
 4. The method according to claim 1,wherein the Lorentz force of the maximum intensity, propagating alongthe axis of propagation, varies by at least 30 N.m⁻³ in thepredetermined time interval between 20 seconds and 10 minutes.
 5. Themethod according to claim 1, wherein the magnetic field is such that theabsolute value of the variation of the density of the maximum Lorentzforce is greater than or equal to 0.05 N.m⁻³.s⁻¹ during saidpredetermined time interval.
 6. The method according to claim 1, whereinthe axis of propagation of the maximum amplitude of the magnetic fieldbelongs to a plane parallel to the direction of casting.
 7. The methodaccording to claim 1, wherein during the casting, the variation in theforce parameter is periodical, the period being between 20 s and 20minutes, or between 1 minute and 15 minutes, or between 2 minutes and 10minutes.
 8. The method according to claim 1, wherein the generators areelectromagnetic inducers, each electromagnetic inducer having a currentflowing through it referred to as induction current, the methodcomprising, during said time interval: a variation in the intensity ofthe induction current; and/or a variation of a frequency of theinduction current; and/or a variation of a distance between anelectromagnetic inducer and the ingot mold.
 9. The method according toclaim 8, comprising a variation in the intensity or in the frequency ofthe induction current flowing through an inducer, the method comprising:a prior step of defining at least one critical value of the intensityand of the frequency of the induction current generating, on a freesurface of the aluminum alloy flowing in the ingot mold, a resonantwave; a determination of a range of variation in the intensity or in thefrequency of the induction current according to said critical valuedefined beforehand.
 10. The method according to claim 9 comprising,during said prior step, a definition of a plurality of critical valuesof the intensity and of the frequency of the induction current, in sucha way as to define a resonance curve, representing the values ofintensity and of frequency generating a resonance of said free surface,the method comprising a determination of a range of variation in theintensity or in the frequency of the induction current in a rangedelimited by said resonance curve.
 11. The method according to claim 1,wherein at least one generator is a permanent magnet, the methodcomprising: a variation in a distance between the permanent magnet andthe ingot mold; and/or a rotation of the permanent magnet, and avariation in the rotation speed of the magnet; and/or a rotation of twopermanent magnets.
 12. The method according to claim 1, wherein thealuminum alloy is chosen from alloys of types 2XXX, 5XXX, 6XXX or 7XXXand wherein the thickness is at least 400 mm or 450 mm.
 13. The methodaccording to claim 1, wherein the dimensionless Hartmann number, at atleast one point of the liquid portion of the alloy, varies at least by afactor of 3, even by a factor of 5, in said predetermined time interval.14. An aluminum alloy ingot obtained by the method according to claim 1.15. The aluminum alloy ingot according to claim 14 having, for anelement of the alloy, of which the content by weight is greater than0.5%, or for the sum of two elements of the alloy of which theindividual content by weight is greater than 0.5%, a dispersioncriterion less than 3.3, preferably less than 3, more advantageouslyless than 2.5, even more advantageously less than 2 and preferably lessthan 1.5, the dispersion criterion being defined according to thefollowing expressions: ε=ΔC_(ZA)/ΔC_(ZR) ΔC_(ZA)=max (C_(ZA))−min(C_(ZA)), ΔC_(ZR)=max (C_(ZR))−min (C_(ZR)), where: max (C_(ZA)) and min(C_(ZA)) respectively designate the maximum and minimum concentrationsof the element considered or of the sum of the two elements consideredmeasured in a zone of analysis, having intermittent macrosegregations,for example between T/2.3 and T/3.3; max (C_(ZR)) and min (C_(ZR))respectively designate the maximum and minimum concentrations of theelement considered or of the sum of the two elements considered measuredin a reference zone, considered as little affected by the intermittentmacrosegregations, for example between T/6 and T/12; said concentrationsbeing measured on at least one profile established at mid-width in avertical plane L/TC and according to a direction TC, said profile beingrepresentative of said intermittent macrosegregations of the elementconsidered according to the direction TC.
 16. The aluminum alloy ingotaccording to claim 14, wherein a spectral intensity criterion is lessthan 0.01, preferably less than 0.007 and preferably less than 0.005,said spectral intensity criterion being calculated by: determining amaximum amplitude of a Fourier transform of a profile representative ofan intermittent macrosegregation of an element of which the content byweight is greater than 0.5% or the sum of two elements of the alloy ofwhich the individual content is greater than 0.5%, the profile beingestablished according to said direction TC, said maximum amplitude beingdetermined in a range of spatial periods between 8 and 25 mm,standardizing said maximum amplitude by a nominal concentration of saidelement or by the sum of the nominal concentrations of the two elementsconsidered.